Refactor DependencyModel, make it ready for tests

test/Microsoft.DotNet.Tools.Tests.Utilities/Mock/EnvironmentMockBuilder.cs

@@ -0,0 +1,48 @@
+// Copyright (c) .NET Foundation and contributors. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+using System.Collections.Generic;
+using Microsoft.Extensions.EnvironmentAbstractions;
+
+namespace Microsoft.DotNet.Tools.Test.Utilities.Mock
+{
+    internal class EnvironmentMockBuilder
+    {
+        private Dictionary<string, string> _variables = new Dictionary<string, string>();
+
+        internal static IEnvironment Empty { get; } = Create().Build();
+
+        public static EnvironmentMockBuilder Create()
+        {
+            return new EnvironmentMockBuilder();
+        }
+
+        public EnvironmentMockBuilder AddVariable(string name, string value)
+        {
+            _variables.Add(name, value);
+            return this;
+        }
+
+        internal IEnvironment Build()
+        {
+            return new EnvironmentMock(_variables);
+        }
+
+        private class EnvironmentMock : IEnvironment
+        {
+            private Dictionary<string, string> _variables;
+
+            public EnvironmentMock(Dictionary<string, string> variables)
+            {
+                _variables = variables;
+            }
+
+            public string GetEnvironmentVariable(string name)
+            {
+                string value = null;
+                _variables.TryGetValue(name, out value);
+                return value;
+            }
+        }
+    }
+}